1. Field of the Invention
The present invention relates to a semiconductor laser drive apparatus using a modulation signal that drives a semiconductor laser to emit light, an optical write apparatus that implements such a semiconductor laser drive apparatus, an imaging apparatus such as a laser printer, a digital copying machine, and a facsimile machine, that implements such an optical write apparatus, and a semiconductor laser drive method using a modulation signal for driving a semiconductor laser to emit light.
2. Description of the Related Art
Technologies relating to the present invention are disclosed, for example, in Japanese Laid-Open Patent No. 2001-88344 and Japanese Laid-Open Patent No. 2002-32140.
Japanese Laid-Open Patent No. 2001-88344 discloses an imaging apparatus such as a digital copying machine that is adapted to form an image on a photoconductor drum by scanning the photoconductor drum with a laser beam output from a semiconductor laser oscillator so as to constantly obtain a stable optical output intensity regardless of a change in the environmental temperature and to thereby form an image with a stable density, in which apparatus light emission level stabilization control is performed on a light emission level of the semiconductor laser oscillator during a non-image formation time and a light emission level of the semiconductor laser oscillator during an image formation time so that both light emission levels may be maintained at their respective predetermined values.
Japanese Laid-Open Patent No. 2002-32140 discloses an imaging apparatus having a cheap and small structure that is adapted to modulate a semiconductor laser using a modulation signal to realize high speed modulation control of the semiconductor laser and acquire an optical quenching ratio, and form an electrostatic latent image on a photoconductor by scanning an optical beam from the semiconductor laser, the apparatus including a first current drive unit that supplies to the semiconductor laser a first current that does not actually cause the semiconductor laser to emit light but causes it to be in a state that allows high speed modulation, which first current is supplied at a first timing based on the modulation signal; and a second current drive unit that supplies to the semiconductor laser a second current that is turned on/off according to the modulation signal, which second current is supplied at a second timing that is later than the first timing based on the modulation signal; wherein the semiconductor laser is arranged to emit light by the combined current of the first current and second current.
In a semiconductor laser (LD), a threshold current (Ith) and an operation current (Iop) change depending on the temperature or the elapsed time, and thereby, a monitor photodiode (PD) that is implemented in the semiconductor laser monitors the amount of light being output and controls the current being supplied to the semiconductor laser so that the amount of light output may remain fixed. Accordingly, in a conventional optical write apparatus that performs on/off control of a semiconductor laser according to image data being input and scans a beam to write an image, a semiconductor laser drive circuit may use the following control methods:    {circle around (1)} supplying the operation current (Iop) with which a predetermined amount of light can be obtained during light-on time, and not supplying the operation current during light-off time.    {circle around (2)} reducing a turn on delay time by supplying a bias current (Ib) during light-off time in order to increase the switching speed.    {circle around (3)} setting a modulation current to a fixed value in order to reduce variations in the turn on delay, and changing the bias current in accordance with a change in the threshold current.
FIG. 1 is a graph illustrating a relation between an applied current in a semiconductor laser drive circuit that performs control according to the above control method {circle around (1)} and an amount of light. FIG. 2 is a circuit diagram showing a circuit configuration of this semiconductor laser drive circuit. According to this arrangement, the semiconductor laser drive circuit includes a semiconductor laser LD, a switch 1, a current source 2, a sample hold circuit 3, an amplifier 4, and a photodiode PD. The switch 1 is operated based on a modulation signal and performs on/off control of a modulation current applied to the semiconductor laser. The current source 2 supplies a drive current to the semiconductor laser LD according to a voltage set by the sample hold circuit 3. The photodiode PD feeds back a light emission quantity of the semiconductor laser LD as a feedback signal to the amplifier 4. According to a sampling signal from an external source, the sample hold circuit 3 samples an output from the difference amplifier 4 that receives the feedback signal from the photodiode PD and a light emission control voltage, and performs APC operations. Then, the APC-produced voltage is applied to the current source 2.
In such an arrangement, it is known that, owing to the required excitation time for the semiconductor laser LD, it takes time ns for the semiconductor laser LD to emit light from the time a current is supplied. This is referred to as turn on delay (refer to FIG. 3 and Understanding Fundamentals and Applications of Semiconductor Lasers; Hirata, Shoji; CQ Publishing Co., Ltd.; 2001). Also, the amplitude of the switching current is relatively large, and it is therefore difficult to increase the switching speed of the switch element (e.g., transistor). Thus, the laser emission rise time and fall time tend to be long.
FIGS. 4 and 5 respectively illustrate a current-light characteristic and a circuit configuration of a semiconductor laser drive apparatus that performs control according to the control method {circle around (2)}. In this method, during light-off time, a bias current with a fixed value is supplied. Thus, a bias current source 5 corresponding to a current source for the bias current is implemented parallel to the switch 1 and the current source 2 for the modulation current as is shown in FIG. 5. Other components of this semiconductor laser drive apparatus are identical to those shown in FIG. 2.
According to this method, the bias current needs to be arranged so that it does not exceed the threshold current Ith even when the threshold current (Ith) changes due to environmental temperature change and variations in the elements, and thus, the bias current cannot take a large value as is shown in FIG. 4. Therefore, the effects of supplying this bias current are not very adequate, and differences in the turn on delay time may occur since switching is performed without taking into consideration the potential differences in the actual threshold currents. Also, in FIG. 4, the threshold current Ith is represented by the intersecting point between the horizontal axis indicating no change in the amount of light and an extension of the line of the current-light characteristic curve at 25° C. with the greater inclination (slope) indicating a greater change rate of the amount of light with respect to the modulation current amount. This threshold current is arranged to be greater than the fixed bias current. The slope of the line intersecting the horizontal axis may also be referred to as the differential quantum efficiency, and this differential quantum efficiency is represented by η.
FIGS. 6 and 7 show a current-light characteristic and a circuit configuration of a semiconductor laser drive circuit that performs control according to the control method {circle around (3)}. According to this method, the bias current source 5 as is shown in FIG. 5 is arranged to supply a threshold current as the bias current (see FIG. 7). Other components of this semiconductor laser drive circuit are identical to those shown in FIG. 5. It is noted that this arrangement corresponds to the first prior art document described above (Japanese Laid-Open Patent No. 2001-88344). In this arrangement, the bias current is controlled to correspond to the threshold current so that turn on delay can be minimized and optimal light emission can be realized.
However, since the threshold current is constantly being supplied according to this arrangement, light may inherently be emitted even when light emission is unnecessary (i.e., during light-off period). Although the inherent light emission may not amount to much, the light is still irradiated onto the photoconductor which may lead to fogging and degradation of the photoconductor drum. Also, as can be appreciated from FIGS. 1, 4, and 6, the differential quantum efficiency (slope) tends to decrease with the increase in temperature; thus, when the modulation current detected at a low temperature is used as the fixed modulation current, at high temperature, the bias current may exceed the actual threshold current thereby increasing the offset light emission.